feat(shell): replace plain-text context indicator with color-coded visual progress bar

[xiaoye5200]

Problem

The current context: 0.0% status bar indicator is plain text with no visual weight, making it hard to gauge context usage at a glance, especially as usage approaches limits.

Expected Behavior

Replace it with a Unicode block progress bar (e.g. Context █░░░░░░░░░ 4%) with color coding:

• Green (< 70%)
• Yellow (70–85%)
• Red (≥ 85%)

Matches the visual style of the claude-hud plugin.

Related PR

PR #1972 is implementing this: #1972